An internal combustion engine has a crank-shaft driven by the connecting rods and pistons, and one or more camshafts which actuate the intake and exhaust valves of the cylinders. The camshaft is connected to a timing gear by means of a timing drive, such as a belt, chain or gears. In a variable cam timing system, the timing gear is replaced by a variable angle coupling, known as a phaser. The phaser is provided with a rotor connected to the camshaft in a housing or stator connected to the timing gear. This allows the camshaft to rotate independently of the timing gear, within angular limits, to change the relative timing of the camshaft and the crank-shaft. The term “phaser”, as used in the present text, includes the stator and the rotor and all of the parts to control the relative angle of position of the stator and the rotor to allow the timing of the camshaft to be offset from the crank-shaft. In any of the multiple-camshaft engines, it will be understood that there could be one or more phasers per engine.
A phaser as described in the introduction is known in the prior art. Most variable camshaft phasers in production today are hydraulically activated devices, using vanes received in recesses, the vanes and the recesses enclosing fluid pockets, wherein the fluid pressure in the fluid pockets will control the angular position of the vane in the recess. The phasers, known in the prior art, are activated by oil pressure derived from an engine oil pump. One example of such a cam phaser is described in U.S. Pat. No. 6,035,816 by Ogawa et al. In this example, Ogawa et al. disclose a cam phaser with vanes received in recesses, the vanes and the recesses enclosing fluid pockets. The cam phaser utilizes an oil pump to feed oil to a control valve which in turn directs the oil to the appropriate fluid pockets in order to control the angular position of the vane in the recess. When oil is evacuated from a pocket, the oil is returned to an oil sump to be picked up by the pump at a later time.
In order to optimise the phaser performance, theoretically the capacity of such an engine oil pump should be as high as possible. However, the bigger the engine oil pump will be, the bigger the parasitic power losses the oil pump will cause. Therefore, a compromise must be found in order not to overlay fuel economy gains of the phaser with losses created by a larger engine oil pump.